Electrostatic means for liquid dispersion in minute droplets are used in a variety of technological applications, such as paint spraying, ionization for chemical analysis, drug inhalation, synthesis of particles from liquid precursors, and surface coating, by way of example and not limitation. The class of atomizers in which the dispersion of the liquid is driven exclusively by electric forces is referred to heretofore as electrospray (ES). Within this class of atomizers it is often desirable to tightly control the size distribution of the resulting aerosol. Such a system can be implemented by feeding a liquid with sufficient electric conductivity through a small opening, such as the tip of a capillary tube or a suitably treated “hole”, maintained at several kilovolts relative to a ground electrode positioned a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up farther downstream into a spray of fine, charged droplets. In view of the morphology of the liquid meniscus, this regime is labeled as the cone-jet mode.
Among the key features that distinguish the cone jet electrospray from other atomization techniques are: quasi-monodispersity of the droplets; Coulombic repulsion of the charged droplets, which induces spray self-dispersion, prevents droplet coalescence and enhances mixing with the oxidizer; and the use of a spray “nozzle” with a relatively large bore with respect to the size of the generated droplets, which implies that liquid line obstruction risks are minimized. The cone-jet mode can produce droplets/particles over a wide size range, from submicron to hundreds of micrometers, depending on liquid flow rate, applied voltage and liquid electric conductivity.
Within the cooling context advancements of integrated circuits have been recently hampered by the severe challenge of the removal of high heat flux. Effective chip cooling may become the bottleneck of further progress in the microelectronic industry. Compared to conventional fan cooling that often rely on a thermal spreader, cooling by direct liquid impingement on the chip back side is promising for high heat flux removal, because it eliminates the contact thermal resistance, promotes high velocity gradients that favour heat dissipation, and exploits the liquid latent heat when phase changes occur. The coolant can take the form of impinging jets or sprays. Micro jets array generated by silicon microfabricated nozzles with open or closed drainage are examples of jet cooling. Spray cooling, currently used in some supercomputers such as the CRAY X-1, in principle is more effective than jet impingement cooling, mainly because the liquid film formed by sprays is typically much thinner (by a factor of ten) than that of liquid jets.
The physical process of spray cooling results from the impact of droplets on a heated surface, which, in turn, may lead to splash, spread, or rebound. If the surface temperature is higher than the Leidenfrost point of the liquid, the droplet tends to rebound because the pressure of the vapour below the liquid partially lifts the droplet. As a result, in conventional sprays only a fraction of the liquid cooling capacity is exploited because of this rebound loss. A possible approach to reduce or even entirely eliminate this loss is to electrically charge the droplets with respect to the hot conducting surface and rely on Coulombic attraction, if charge leakage on contact is sufficiently slow. In this context, ES is potentially well suited for cooling purposes because of its unique properties described earlier, especially the small and uniform droplet size and reasonably even number density throughout the spray. The ES-generated droplets are small and quasi-monodisperse, with reasonably uniform number density throughout the spray. The inner diameter of the ES nozzle is typically 10-100× larger than the droplet, which reduces the risk of clogging and dramatically decreases the liquid pressure drop, from ˜105 Pa of a conventional atomizer to ˜103 Pa of ES systems.
ES has been widely used in ionization mass spectroscopy. In virtually all other applications, it has been plagued by one critical drawback: the low flow rate of a single ES source, which would make it inadequate even for spray cooling. This drawback has been recently overcome by microfabricated multiplexed ES (MES) systems, which allow for the dispersion of large flow rates through multiple, densely packed ES sources operating in parallel. Furthermore, the “digital” version of the MES devices, in which each individual spray can be turned on/off selectively via electronic control, has also the potential for precise local thermal management of hot spots on microelectronic chips.
It is an object of this invention to provide an improved electrospray apparatus and method which enables production of electrically charged droplets of highly uniform size from multiplexed electrosprays for the cooling of microelectronic chips or other surfaces requiring precise control of the thermal load. As a proof of concept, a miniaturized MES cooling device was demonstrated to remove a heat flux of 96 W/cm2, with the potential of additional scale up, and with an unprecedented cooling efficiency reaching up to 97%.